Field of the Invention
The present invention relates to a method for growing Group III nitride semiconductor crystal on a mesa-formed substrate and a Group III nitride semiconductor.
Background Art
When a Group III nitride semiconductor light-emitting device is produced by crystal growth of Group III nitride semiconductor on a sapphire substrate, a technique has been developed to form mesas on a Group III nitride semiconductor growing surface of the sapphire substrate to improve the light extraction performance (e.g. Japanese Patent Application Laid-Open (kokai) Nos. 2004-200523 and 2005-101566).
Japanese Patent Application Laid-Open (kokai) No. 2003-526907 discloses that a mesa stripe pattern is formed on a sapphire substrate, a GaN is vertically grown on the top surface of the mesa, and further laterally grown, thereby obtaining a GaN semiconductor having a low threading dislocation density.
However, when Group III nitride semiconductor is grown on the mesa-formed sapphire substrate by these methods, voids are formed on side surfaces of the dents or mesas, resulting in a problem that crystallinity and flatness of Group III nitride semiconductor are deteriorated.
Therefore, Japanese Patent Application Laid-Open (kokai) No. 2003-318441 discloses that sides in planar view of dents or mesas intersect with an a-plane of the sapphire substrate. When the dents or mesas are formed in such a manner, GaN is grown on the top surfaces of the mesas of the sapphire substrate and the bottom surfaces of the dents, and GaN is difficult to grow on side surfaces of the mesas or dents. As the growth progresses, the crystals are combined together, thereby obtaining GaN having superior crystallinity and high flatness, with no void formed therein. On the other hand, when the sides in planar view of the dents or mesas are parallel to the a-plane of the sapphire substrate, lateral growth of the GaN grown on the dents or mesas is slow. Therefore, a space near the side surfaces of the dents or mesas is difficult to be buried with GaN, and the surface flatness of GaN is deteriorated.
Japanese Patent Application Laid-Open (kokai) No. 2011-77265 discloses that all side surfaces of mesas on a substrate having an a-plane or c-plane main surface are the surfaces which inhibit the crystal growth of Group III nitride semiconductor. It is also disclosed that when the mesas are hexagonal prisms, the angle between each side of hexagons in planar view and the m-axis is 15°.
When the Group III nitride semiconductor is grown in a direction perpendicular to the main surface from the top surfaces of the mesas and the bottom surfaces of the dents, there is an advantage that the threading dislocation density is reduced on the most upper surface of the grown semiconductor because the threading dislocation is bent laterally through the facet growth. However, there is a disadvantage that the threading dislocation density is uneven on the upper surface of the grown semiconductor layer because the laterally bent threading dislocation is concentrated in the vicinity of a boundary between the dents and the mesas.
When the Group III nitride semiconductor is vertically grown on the top surfaces of the mesas and the bottom surfaces of the dents on the mesa-formed substrate, steps need to be buried by laterally growing the Group III nitride semiconductor. That is, after the Group III nitride semiconductor was grown under the growth conditions where growth is faster in a direction perpendicular to the main surface of the substrate, it is necessary to grow the Group III nitride semiconductor under the growth conditions where growth is faster in a lateral direction. Therefore, there is a problem that the production method is complicated.